According to wildlife charity WWF, South Africa has been the focal point of poaching because it has the largest population of the threatened animals in the world.

The animals are being illegally hunted to supply the rhino horn market, particularly in Vietnam, driven by a belief it can cure cancer.

The grim news from South Africa comes just days after WWF confirmed rhinos had become extinct in Vietnam, where a small population of critically endangered Javan rhinos had been hanging on.

The failure of efforts to conserve the Javan rhinos in Vietnam, where the last individual was found dead with its horn missing and a bullet in its leg, means there is thought to be just one group of less than 50 individuals remaining in the world, in Indonesia.

Dr Christy Williams, WWF's Asian rhino expert, said: "The unfounded rumour that rhino horn can cure cancer most likely sealed the fate of the last Javan rhino in Vietnam.

"This same problem is now threatening other rhino populations across Africa and South Asia."

Tom Milliken, rhino programme co-ordinator for Traffic, the wildlife trade monitoring network, said: "It's tragic that the Javan rhino has been wiped out in Vietnam by the same forces that are driving rhino poaching in Africa.

"This is the ultimate wake-up call for the Vietnamese government to turn aggressively on its internal rhino horn market."